The shores of Lake Pukaki offer clues to the formation of the landscape. In this embankment, which has been exposed through erosion, many layers of sediments deposited at various times and under various conditions of the area are shown. The layers, which were deposited indirectly through glacial action, are not the typical glacial deposits of moraines. These beds were formed in fluvial outwashes from the glaciers as they expanded and retreated at various times.

Dan Bailey's Geo Postcard

Greywacke and Shale Road Cut

The above photo is of a road cut along State Route 73 to the east of the Southern Alps. This road cut consists of greywackes and shales of the Torlesse Supergroup. These rocks are sedimentary rocks resulting from the lithification rock fragments and sand in a clay matrix that made up the original New Zealand geosyncline. This sedimentary rock was then subsequently uplifted, twisted and turned, and brought to the surface by the Rangitata and Kaikoura orogenies.The rocks in this road cut showed several interesting sedimentation features. This rock outcrop showed graded bedding. The graded bedding arose from variable velocity underwater or marine mudflows that deposited sand, silt, and clays in layers that were graded from the coarsest/densest grains to the finest/lightest clay sediments. This mechanism of deposition gives rise to sole markings, where the densest/largest debris is dragged along the bottom of a flow gouging the freshly deposited mud layer below it. These gouges or scours are then infilled by coarser sediments, yielding a “cast” of the original marking. Once the sediments are lithified and the shale layer is eroded away, the sole mark becomes visible. The specific type of sole marking shown in the above photo is a symmetric grove cast, which can be used to determine the trend in flow direction.The graded bedding can be seen with the dark fine clay grains (almost black in the lower right) above the lighter brown coarser sand grains (in the middle) with another dark clay layer below in the upper left. The prominent ridge seen in the coarse brown layer is a grove cast indicating the trend in direction of the mudflow that deposited these sediments. The whole rock formation was uplifted from its horizontal orientation of deposition to its current nearly vertical orientation on the road cut.

Kura Tawhiti Conservation Area

The above photograph was taken at the Kura Tawhiti Conservation Area also along State Route 73 just east of the Southern Alps. The photo shows a rock formation that is known as a tor. A tor forms when a continuous layer of rock is eroded and weathered, leaving the most resistant rocks protruding from the landscape. Tors are usually made of granite or weather resistant metamorphic rocks. What makes this rock outcrop so interesting is that it is made out of limestone, which is particularly soft and susceptible to chemical weathering. This rock started as a continuous layer of sedimentary limestone. The layer became exposed during the last mountain building event and was chemically and freeze-thaw weathered in the rounded/scooped manner characteristic of limestone. This rock outcrop must be relatively young for two reasons. First, limestone is a soft rock that is susceptible to chemical weathering and would be completely eroded away if it were exposed for too long. Second, glacial activity during the last glacial maximum would have destroyed this susceptible surface formation. Therefore, is must be younger than about 10,000-12,000 years old. The rock outcrop is now a popular attraction for tourism and bouldering (type of rock climbing characterized by short, technical climbs without a rope).

Liz Nyberg -Geo postcard

Lady Know Geyser: This geyser, located at wai-o-tapu (sacred water), was discovered in 1901 by prisoners. These newly relocated prisoners were allocated the job of clearing and planting pine trees. The prisoners used the geyser to wash their clothing when they found out that they could make the geyser erupt by adding soap. The cone shape of the geyser was formed by silica accumulating on rocks placed around the geyser base to make the water spray higher. The natural eruption cycle is every 48 to 72 hours, however, when artificially induced it erupts daily at a designated time. The geyser is caused by a heated reservoir underground comprised of 2 chambers. The bottom chamber is 150 degrees C and the upper chamber does not exceed boiling. The upper chamber acts as a lid to keep the hot water down. When soap is added, it softens the water in the upper chamber and it mixes with the lower chamber causing an eruption that can reach up to 20 meters and last for up to an hour depending on weather.
Fox Glacier: This glacier drains from Mt. Cook and flows toward the west coast of New Zealand. Characteristics of this glacier include the seracs or the towers of ice formed on top of the glacier and trim lines from former lateral moraines. There is also melt water at the bottom of the glacier that is produced from midglacial tunnels in the middle of the glacier. There is dead ice further down the valley that is evidence of glacial recession. Dead ice are blocks of ice that have been left behind by a retreating glacier and were insulated by the outwash debris. Occasionally glaciers can get clogged and when finally released it causes a huge flood. Surge glaciers happen when too much pressure builds up and force the glacier to move abnormally fast. This glacier is advancing due to accumulation of increased snowfall (ablation) which is shown by the layering of dust bands.

Chi Poon - Geo Postcard

Geyser: Lady Knox Geyser in Rotorua. It was discovered 100 years ago. The structure of the geyser is very unique. It contains a structure that contains two chambers; the top one is occupied by cold water and the bottom camber is filled with hot water about 150°C. These two chambers are separated by a really narrow tube that put a restriction on the mixing of the cold and hot water, so these water chambers remain unmixed and the cold water serves as a lid and put pressure on the hot water making it super-heated.
On the day of our visit, the manager put on a show for us and makes the geyser erupt steam and water (otherwise this geyser has a various natural cycle eruption of 24-72 hours). He puts soap into the geyser, which then generates foam on the cold water lowering the pressure that the cold water put on the bottom hot water chamber allowing the lower chamber hot water to push its way up mixing with the cold water and ultimately erupts steam and water. You might think the erupted water would be quite hot, but it mixes with a lot of cold water and the erupted water is relatively cool. This eruption can last an hour or two, and then eventually the water remaining in the geyser would cool back and the eruption would end, and the heated ground water would slowly to refill the bottom chamber.


South Crater, the Crater Imposter: During the hike of Tongariro, which is a composite volcano (alternating layering of lava and rock fragments). This is a photograph of a crater imposter called the South Crater. It is amazing how the basaltic lava flow naturally formed a crater like rim, so perfectly surrounds an area, which trapped rainfall and snow melt, structuring the area into a sediment basin, which is quite flat. The rainfall and snow melt erode and transport the slope and deposit the more well sorted, finer grained and more rounded sediments at the basin. The “crater” occasionally fills up with snowmelt and rainfall. As you can see in the photograph a little pond of water can still be seen on the sediment basin.

Jon Campano - Geo Postcard

Pictured is a mud pool located at Wai-O-Tapu on the North Island of New Zealand. The water level at this mud pool changes over time, having risen in the last two years. Layer upon layer of ash containing fragmented glass make up much of the mud pool. These layers of ash are unstable and alter to a clay after being exposed to hot water. This hot water is produced from underground magma (which in turn also provided the needed materials for a gold deposit to be formed at Martha Hill). The clay, called smectite, expands with the new heat. At this particular mud pool, the heat source is coming from the Hikurangi Trough subduction zone.
As time went on, volcanic activity lessened as the volcanic hot spot slowly migrated southeast, passing through Tauranga to be presently centred around the North Island.




Pictured are tors located on the South Island of New Zealand. Tors are towers of rock of massive size which are formed by chemical and physical weathering, usually either freeze-thaw weathering or groundwater weathering before exposure. Typically tors are meade of granite, but here they are made of limestone, which is found rarely in New Zealand. The evidence tells us the tors are indeed make of limestone: they were weathered in a scalloped surface, particles can be seen, limestone tends to weather in rounded shapes and they have a corroded appearance.
They have formed geologically fast, after the last ice age approximately 12 thousand years ago, during the most recent mountain building period.
These tors are remnants of what was once a horizontal layer. Tear-pants weathering, also called honeycomb weathering, can be seen on the tors. You can see Dan Bailey on the top right of the tor to illustrate the massive scale.

Po-Chedley Geology Postcards

The Champagne Pool



This steaming lake is situated in a crater that exploded 700 years ago. The crater is 62 meters deep with steep sides and extremely hot water. Water enters the lake from below with a temperature of 230 degrees centigrade (446 degrees Fahrenheit) and cools to about 74 degrees centigrade (165.2 Fahrenheit), which explains the hot, steaming surface water. The orange hue of the lake is from the mineral antimony contained in the lake. Interestingly, the lake also contains traces of arsenic, mercury, thallium, gold, and silver. The source of the intense heat in the subterranean streams is magma left from past eruptions. This intense heat allows water as hot as 300 degrees centigrade (572 Fahrenheit) to absorb minerals from the rocks and transport them throughout the hydrothermal system. The other side of the Champagne pool has an array of water colors called the "Artist's Pallet" derived from elements and minerals in the water such as colloidal sulfur, manganese oxide, silica, iron oxide, and carbon. A sulfur smell is all around the area from hydrogen sulfide.


Lake Pukaki and Mount Cook



This incredible view is from a lookout point on the edge of Lake Pukaki toward New Zealand's highest mountain - Mount Cook. Mount Cook rises 12,316 feet and is still uplifting today. The photograph demonstrates the striking light blue color of Lake Pukaki, which is from light scattering of fine grained particles such as glacial flower (microscopic grains of rock produced from grinding glaciers) and calcium content. Lake Pukaki is 50 - 60 kilometers long and 100 meters deep. The lake was formed over the last four (or more) glacial cycles and has accumulated 400 meters of mud during interglacials (warm periods). During a glacial cycle, such as the major glacial cycle 17,000 years ago, the ice was likely 600 to 700 meters thick. The basin in which the lake sits was carved out by erosion of massive glaciers and fills in with sediment during interglacial periods keeping the lake level relatively shallow (other glacial "finger lakes" are hundreds of meters deep). The lake level today is artificially raised 10 to 20 meters for hydroelectric power generation.

Geo-Postcards from New Zealand (Nov. 16-30)